It is often desirable for a physician to be able to monitor a patient's respiration rate over relatively long periods of time. Accurate monitoring of respiration rate is especially important in the case of infants suspected to be susceptible to crib death or sudden infant death syndrome (SIDS). Children believed to be at a high risk of SIDS are often monitored with various types of respiration monitoring devices which are designed to detect apnea, or the cessation of breathing.
One of the most common commercially available apnea monitors is based on an impedance pneumograph. This device operates by creating an alternating current signal between electrodes mounted on the thorax. Respiration is detected by measuring a change in the impedance between the electrode as the chest expands. Specifically, this device is based on a correlation between the impedance measured between the electrodes and the respiratory tidal volume in the non-moving subject, whereby changes in subject position affect the amplitude of the detected signal. These types of devices, however, have been found to be inaccurate because certain movements can occur in a non-breathing subject which produce a signal very similar to a respiratory waveform.
Another electromechanical respiration monitoring device is based on a magnetometer which measures respiration by correlating the distance between electrodes mounted on the chest and back of the patient. Although this device can be used to measure respiration, it is extremely sensitive to motion artifacts and to the erroneous detection of respiration due to spurious signals. In addition, both the magnetometer and the impedance type monitor often give erroneous indications of respiration because the chest cavity will expand when the patient attempts to breathe even though there may be an obstruction which prevents the actual flow of air.
In recent years, there has been considerable interest in the development of a respiration monitor capable of measuring respiration rate from a patient's breath sounds. One of the major problems that has prevented the development of a respiration rate monitor based on breath sounds, however, is that of identifying breath sounds in the presence of other, unrelated sounds. These other sounds may be generated by the subject (e.g., talking, snoring, abdominal sounds, muscle sounds and heart sounds) or external to the subject (e.g., machine noise, talking by other people, radio and television noise, footsteps and doors closing). Subject-generated noise, the main source of which is heart sounds, is difficult to control or monitor independent of breath sound noise. The respiration rate monitor provided by the present invention overcomes these difficulties, as discussed in greater detail below.